(OO.o or OOo), commonly known as OpenOffice, is an office application suite available for a number of different computer operating systems. It is distributed as free software and written using the gtk toolkit. It supports the ISO/IEC standard OpenDocument Format (ODF) for data interchange as its default file format, as well as Microsoft Office formats among others. As of March 2009, OpenOffice supports over 80 languages. was originally derived from StarOffice, an office suite developed by StarDivision and acquired by Sun Microsystems in August 1999. The source code of the suite was released in July 2000 with the aim of reducing the dominant market share of Microsoft Office by providing a free and open alternative; later versions of StarOffice are based upon with additional proprietary components. pioneered the ISO/IEC standard OpenDocument file formats (ODF), which it uses natively, by default. It also supports reading (and in some cases writing) many legacy and current proprietary file formats (e.g.: WordPerfect through libwpd, StarOffice, Lotus Software, MS Works through libwps, Rich Text Format), most notably including Microsoft Office formats. Since version 3.0 the default format of is based on draft versions for OASIS ODF 1.2. The OpenDocument specification was "approved for release as an ISO and IEC International Standard" under the name ISO/IEC 26300:2006.

Microsoft Office inter-operability has published extensive documentation from their analysis of the closed Microsoft Office binary formats.

Sun Microsystems has developed an ODF plugin for Microsoft Office which enables users of Microsoft Office Word, Excel and PowerPoint to read and write ODF documents. The plugin, as of October 2008 works with Microsoft Office 2003, Microsoft Office XP and Microsoft Office 2000.